Polyalpha-olefins (PAOs) comprise a class of hydrocarbons manufactured by the catalytic oligomerization of olefins such as C2-C20 alphaolefins. PAOs have achieved an importance in the lubricating oil market as they can be used as basestocks useful for synthetic lubricants. With this application in mind, the research efforts have generally focussed on fluids exhibiting useful viscosities over a wide range of temperature, such as improved viscosity index (VI). The viscosity index is an empirical number which indicates the rate of change in viscosity of an oil within a given temperature range. High viscosity index is usually desirable because the oil will have higher viscosity at higher temperature, which translates into better lubrication and better protection of the contacting machine elements. The viscosity index (VI) is calculated according to ISO2009.
Processes for producing PAOs are known from the art. The oligomerization of PAOs in presence of metallocene catalysts has been described for example in WO02/14384. This document discloses the use of CpFlu catalysts for the oligomerisation of PAO. CpFlu catalysts are metallocene catalysts incorporating a cyclopentadienyl-fluorenyl structure. Comparative examples J and K are directed to the use of racemic bridged metallocene catalysts such as rac-ethylbis(indenyl)zirconium dichloride or rac-dimethylsilyl-bis(2-methyl-indenyl)zircomium dichloride in combination with methylalumoxane (MAO) at 40° C. (at 200 psi hydrogen or 1 mole of hydrogen) to produce isotactic polydecene reportedly having a KV100 of 702 cSt, and a viscosity index of 296; or to produce polydecene reportedly having a KV100 of 1624, and a viscosity index of 341, respectively.
In several applications it is preferred to obtain PAOs with a low kinematic viscosity at 100° C. (KV100). Thus it is looked for PAOs with a low KV100 combined with a good viscosity index. Such compromise may be achieved in promoting the production of 11-octyldocosane within the trimer fraction. Indeed, the presence of this component is beneficial for fluid property, as it allows better viscometrics (KV100 and viscosity index). WO2007/011459 discloses the oligomerisation of alpha-olefin into a PAOs containing 11-octyldocosane within the trimer fraction. Among others, the use of rac-dimethysilylbis(tetrahydroindenyl)zirconium dichloride in an oligomerisation process without the addition of hydrogen during the oligomerization step is disclosed. Also it discloses obtaining oligomers with high degree of unsaturated bonds. However, it does not disclose obtaining oligomers with kinematic viscosity KV100 of at most 10 cSt as measured according to ASTM D445 at 100° C. together with a viscosity index of more than 160.
Another problem encountered in the prior art is to conduct oligomerization reactions with a good catalyst activity. Indeed, it has been observed that the production of low kinematic viscosity products is accompanied by a loss in the catalyst activity.
Thus there is still a need to produce PAOs having good viscometrics (i.e. a good compromise between kinematic viscosity and viscosity index) together with a good catalyst activity.